Nematodes are elongated symmetrical roundworms that constitute one of the largest and most successful phyla in the animal kingdom. Many nematode species are free-living and feed on bacteria, whereas others have evolved into pests or parasites of plants and animals, including humans.
Nematode pests of plants are responsible for many billions of dollars in economic losses annually. Nematode plant pests feed on stems, buds, leaves and, in particular, on roots of more than 2,000 vegetables, fruits, and ornamental plants, causing an estimated $100-125 billion crop loss worldwide. Nematodes are present throughout the United States (US), but are mostly a problem in warm, humid areas of the south and west, as well as in sandy soils. The most economically damaging plant nematode pest genera belong to the family Heterderidae of the order Tylenchida, and include the cyst nematodes [genera Heterodera and Globodera, e.g., soybean cyst nematode (Heterodera glycines, SCN) and potato cyst nematodes (G. pallida and G. rostochiensis)], and the root-knot nematodes (genus Meloidogyne).
Root-knot nematodes infest thousands of different plant species including vegetables, fruits, and row crops. Cyst nematodes are known to infest tobacco, cereals, sugar beets, potato, rice, corn, soybeans and many other crops. Heterodera schachtii (BCN) principally attacks sugar beets, and Heterodera avenae is a pest of cereals. Heterodera zeae feeds on corn, and Globodera rostochiensis and G. pallida feed on potatoes. The soybean cyst nematode (SCN) is present in every soybean-producing state in the US, and causes total soybean yield losses estimated to be nearly $1 billion per year. Once SCN is present in a field, it cannot feasibly be eradicated using known methods. Although soybean is the major economic crop attacked by SCN, SCN attacks some fifty hosts in total, including field crops, vegetables, ornamentals, and weeds.
Cotton root knot nematode (RKN) is a destructive nematode, which forms galls on the roots of cotton plants. The causative agent is Meloidogyne incognita (Kofoid and White) Chitwood, a nematode which can infest a variety of plant species. Nutrient and water uptake are decreased in infested plants, and plants may become susceptible to pathogens, especially Fusarium wilt. Consequently, yield is decreased in plants infested with RKN. In the US alone, an estimated 10.93% of cotton yield loss in 2004 was attributed to RKN (Blasingame and Patel, Proceedings of the Beltwide Cotton Conferences 1:259-262 (2005). RKN is wide-spread throughout the U.S. Cotton Belt. Methods to mitigate RKN damage include rotating cotton crops with non-susceptible crops and application of costly nematicides. However, the most effective way for cotton growers to reduce yield loss and crop damage due to RKN is to grow RKN resistant cotton cultivars.
Signs of nematode damage include stunting and yellowing of leaves, as well as wilting of the plants during hot periods. However, nematodes, including SCN, can cause significant yield loss without obvious above-ground symptoms. For example, an infestation of SCN to a plant can result in dwarfed or stunted roots, decrease the number of nitrogen-fixing nodules on the roots, and/or make the roots more susceptible to attack by other soil-borne plant pests or pathogens.
In contrast to many viral and bacterial pathogens, little is known about the molecular basis of the nematode-plant interaction, limiting the available approaches useful in controlling nematodes. Chemicals useful in controlling nematode plant pests include organophosphates and carbamates, the oldest extant class of nematicides, which target acetylcholinesterase. Imidazole derivatives such as benzimidazole exert their nematicidal effects by binding tubulin. Levamisole acts as an agonist on the nicotinic acetylcholine receptor, and avermectins act as irreversible agonists at glutamate-gated chloride channels. Unfortunately, there are certain debilitating nematode infestations which are difficult, if not impossible, to eradicate with existing control measures. In addition, the currently available nematode control agents have drawbacks in terms of efficacy, expense and environmental safety. For example, methyl bromide, which is an effective pre-plant soil fumigant used to control nematodes in many high-input, high-value crops in the US, is being phased out due to environmental and human health concerns. However, because methyl bromide has provided a reliable return on investment for nematode control, many growers of high value crops may be negatively impacted if effective and economical alternatives are not identified. In addition, environmental concerns, primarily groundwater contamination, ozone depletion, and pesticide residues in food have prompted the removal of Aldicarb, DGBCP, and other toxic nematicides from the market by the US Environmental Protection Agency. Physical control measures (such as solarization and hot water treatment), biological control measures (e.g., crop rotation), and integrated approaches have been used to ameliorate the damage caused by plant nematode pests, but no single method or combination of measures is uniformly effective.
Nematode resistant germplasm and transgenic plants have also been considered as alternatives or complements to chemical control measures. For example, transgenic plants expressing a protease inhibitor shows some resistance to cyst and root-knot nematodes (Urwin et al. 1997. Plant J. 12:455-461). Use of such alternative control measures requires a greater knowledge of the nematode-plant interaction to achieve satisfactory results. Several studies have generated gene expression data suggesting that many host plant genes are up- or down-regulated in response to nematode invasion (Szakasits et al. 2009. Plant J. 57:771-784; Puthoff et al. 2003. Plant J. 33:911-921; Bethke et al. 2009. Proc. Natl. Acad. Sci. 106:8067-8072; Stepanova et al. 2007, Plant Cell 19:2169-2185 and Kilian et al. 2007. Plant J. 50:347-363). However, none of these studies aid the skilled person in predicting which, if any, such genes could be successfully utilized in controlling nematodes, particularly in chimeric gene constructs for deployment in a transgenic plant.
Accordingly, the invention overcomes the deficiencies in the art by providing compositions and methods comprising recombinant nucleic acid molecules and their encoded polypeptides for control of nematode pest infestations in plants.